In a typical TDM system, a transmitter samples pulse signals of relatively low pulse repetition frequency from various data sources or channels and interleaves them with one another to form an aggregate data stream that is transmitted by a high speed aggregate channel to a remote receiver. Ordinarily, the TDM transmitter inserts the signals representative of a single bit or a single character in a single time slot in the aggregate data stream and interleaves the signals from different channels on a bit-by-bit or character-by-character basis so that adjacent time slots contain signals from different channels. However, different size blocks of signals can be used if desired. At the receiver, the individual bits or characters are separated from one another and allocated to various low frequency data channels similar to those at the transmitter.
To permit proper decoding of the data stream at the receiver, the transmitter interleaves the signals from the various data channels in accordance with a fixed schedule which it repeats endlessly and the receiver uses the same schedule to decode the data stream. Each cycle of the schedule is called a frame or an aggregate frame. In addition to data signals, each frame ordinarily includes synchronization signals called frame sync words and various control signals, both for individual channels and for the entire TDM system. Typically, the synchronization and control signals take up a small portion (less than 5%) of the total frame which is referred to as the overhead. To simplify the generation of the signals used to select the particular data channel from which a bit or character is to be transmitted, it is customery to sample the data channels in a fixed pattern which is repeated numerous times within each frame. Each such cycle of repetition is called a subframe.
While the control signals take up a relatively small portion of the aggregate frame, transmission of these signals causes numerous problems in the efficient operation of the TDM.
Because they are part of the overhead, the transmission of control signals reduces the efficiency of data transmission. In an effort to achieve higher efficiency, system designers tend to design frames which do not provide for enough control signaling. As a result, channels with lots of control signal activity are likely to monopolize the available space in the frame for control signaling and/or the channel response to control signaling is likely to be slow.
Further, in prior art systems which use a frame comprising a large number of identical subframes and one relatively small non-repeating portion, the practice is to allocate control signaling to one or more groups of contiguous time slots in the non-repeating portion of the frame and to assign each of these time slots to a specific channel. Such an approach, however, ignores the differences in control signaling activity that are almost certain to exist between channels. In addition, since there is very little data transmitted in the non-repeating portion of the frame, channel data accumulates in the individual channel buffers while the control signals are being sent. While large buffers can be used to eliminate any possibility of data loss as a result of such buffer slippage, the use of large buffers increases the time it takes to send a signal from one end of the system to another. In looped keyboard display systems where a keystroke entered at a local keyboard is displayed on a local CRT by a remotely located computer, the delay in sending the keystroke to the computer and back to the display may be intolerable.